1. Field of the Invention
Exemplary embodiments of the present invention generally relate to a belt device and an image forming apparatus incorporating the belt device, and more particularly, to a belt device having a belt cleaning unit to clean a surface of an endless belt member in rotation by removing residual toner remaining on the surface of the endless belt member by a cleaning roller, and an image forming apparatus incorporating the belt device.
2. Description of the Related Art
In recent years, in accordance with rising demand for higher resolutions and higher image quality, toner produced by polymerization has come to be used in place of conventional toner produced by pulverization. This is because the polymerization method can uniformly confine toner particle size within a narrow range and moreover can obtain particles of high sphericity, and therefore can provide superior reproducibility of even the small dots that correspond to high resolution. On the other hand, toner produced by polymerization can be hard to remove from a cleaning target using the typical blade cleaning method employing a cleaning blade, because the very smallness and sphericity of the toner particles allow them to slip through a small gap formed between the cleaning blade and the cleaning target member.
One solution proposed in Japanese Patent Application Publication No. 2009-020249 (JP-2009-020249-A1) involves a cleaning device that uses an electrostatic cleaning method, by which even toner produced by polymerization can be removed from a cleaning target member. Specifically, the cleaning device in JP-2009-020249-A1 includes a cleaning brush roller that rotates to contact a drum-shaped image carrier that serves as a cleaning target member, a collection roller that rotates while contacting the cleaning brush roller, and a scraping blade that scrapes away residual toner while contacting the collection roller. The cleaning brush roller includes a rotatably supported roller shaft and a brush roller portion formed by multiple fibers attached to the circumferential surface of the roller shaft. A voltage for cleaning (hereinafter, “cleaning voltage”) that has a polarity opposite that of a regular charging polarity of the toner is applied to the cleaning brush roller. A voltage for collecting toner (hereinafter, “collection voltage”) that has the same polarity as the cleaning voltage but which is larger than the cleaning voltage is applied to the collection roller.
Residual toner that remains on a surface of an image carrier after an image transfer process is electrostatically transferred onto the brush roller part by an electrical field formed between the image carrier and the brush roller part of the cleaning brush roller while being scraped by the brush roller part of the rotating cleaning brush roller. Then, after being electrically transferred from the brush roller part of the cleaning brush roller onto the collection roller, the residual toner is scraped from the surface of the collection roller by the scraping blade. Accordingly, toner produced by polymerization can be removed more successfully by the above-described electrostatic cleaning method, compared to removal by the blade cleaning method.
However, the electrostatic cleaning method has a drawback in that it fails to clean belts as well as it cleans rollers.
The present inventors have studied the causes of the above-described problem and found that it is necessary to suppress the ripples in the belt and consequent vibration caused by slidably contacting the cleaning brush roller against the belt, which is wound around multiple tension rollers, while pressing the cleaning brush roller relatively hard against the belt for effective cleaning. For this reason, ordinarily the cleaning brush roller is contacted against the belt to form a cleaning nip not over the entire length of the belt but only at that portion of the belt where the belt winds around the tension roller, that is, where ripples in the belt and consequent vibration cannot occur.
At the same time, however, to achieve a certain level of transferability and sheet attracting ability, an image forming apparatus generally employs a belt member having some electrical resistance. To successfully clean a belt member having some electrical resistance, it is known that a certain amount of electric current for cleaning (hereinafter, “cleaning electrical current”) must be sent in a path from the cleaning brush roller to a tension roller via the belt member. In the cleaning nip, however, when an excessive amount of cleaning electric current flows around toner on the surface of the belt member, an electrical charge that has a polarity opposite the regular polarity of the toner is injected, causing the toner to be charged to the opposite polarity. According to the opposite charge polarity of toner, when the belt member is cleaned, the cleaning ability can be degraded.
A detailed description is now given of the toner charged to the opposite polarity.
FIG. 1 illustrates an enlarged diagram of an example of a cleaning nip of a conventional cleaning device.
As illustrated in FIG. 1, an intermediate transfer belt 901 that serves as a belt member to rotate endlessly is wound around a roller 902 and multiple other rollers, not illustrated. The roller 902 contacts the inner surface of the intermediate transfer belt 901 over the entire circumferential area of the intermediate transfer belt 901, in an area having a belt wound width W1. At the same time, a cleaning brush roller 903 contacts the outer surface of the intermediate transfer belt 901 to form a cleaning nip therebetween. An entire nip width W2 is a length of the cleaning nip in the belt moving direction of the intermediate transfer belt 901 over which the cleaning brush roller 903 scrapes the residual toner from the outer surface of the intermediate transfer belt 901, and is greater than the belt wound width W1.
The cleaning brush roller 903 is rotated by a driving unit, not illustrated, to rotate in a counter direction, that is, against a direction of rotation of the intermediate transfer belt 901 in the cleaning nip, and slidably contacts the outer surface of the intermediate transfer belt 901. Residual toner remaining on the outer surface of the intermediate transfer belt 901 is scraped away by the cleaning brush roller 903 to which a cleaning bias that has a polarity opposite that of toner is applied.
At both an upstream end and a downstream end of the cleaning nip, the cleaning brush roller 903 contacts a tensioned belt area of the intermediate transfer belt 901 where the intermediate transfer belt 901 is not held in contact with the roller 902. (Hereinafter, the belt wound area where the cleaning brush roller 903 contacts the belt tension area on an upstream side of the nip is referred to as an upstream tensioned nip area.) In this upstream tensioned nip area, because the cleaning brush roller 903 contacts the outer surface of the intermediate transfer belt 901 but the cleaning opposite roller 902 does not contact the inner surface of the intermediate transfer belt 901, the cleaning electrical current described above does not flow sufficiently.
By contrast, in the belt wound area where the roller 902 contacts the intermediate transfer belt 901 or in the area indicted by the belt wound width W1, the cleaning electrical current can flow well. Specifically, a large amount of electrical current flows more in the vicinity of a center part of the belt wound area where the nip pressure is greatest, than at the ends of the belt wound area. Accordingly, when toner comes to the center part of the belt wound area, toner can be charged to the opposite polarity easily.
Therefore, to obtain good cleaning ability, of the entire area in the belt moving direction in the cleaning nip, it is necessary to transfer substantially all of the residual toner onto the cleaning brush roller 903 in an upstream tensioned nip area (indicated by an upstream nip width W3) and in the vicinity of an entrance to the belt wound area (indicated by the belt wound width W1).
However, in the vicinity of the upstream tensioned nip area indicated by the entire nip width W2, only the tip parts, that is, the leading edges of fibers forming the cleaning brush roller 903 contact the intermediate transfer belt 901. It is difficult to transfer residual toner onto the fibers under the above-described condition.
The desired transfer of residual toner onto the cleaning brush roller 903 can occur when the fibers of the cleaning brush roller 903 bend at a sharp angle to the intermediate transfer belt 901 and the sides of the fibers of the cleaning brush roller 903 contact the intermediate transfer belt 901. However, as can be seen in FIG. 1, the fibers are inclined as described above only in a small area that is relatively isolated from the entrance over the upstream tensioned nip area indicated by the upstream nip width W3. Further, over the entire belt wound area indicated by the belt wound width W1, only in a small area near the entrance is the toner is not charged to the opposite polarity by the cleaning electrical current. As a result, residual toner left un-transferred to the cleaning brush roller 903 enters the center part of the belt wound area in the conventional cleaning device, causing cleaning failure.
It is to be noted that the cleaning brush roller 903 is used as a cleaning rotating member to explain the above-described problems. However, a structure not having a brush like the cleaning brush roller 903 can also be used to supply a relatively large amount of cleaning electrical current to the belt wound area in the cleaning nip. As a result, in the cleaning nip, the residual toner further remaining in the upstream tensioned nip area can enter the belt wound area, which is likely to cause cleaning failure.
Further, the cleaning brush roller 903 in the above-described structure applies a cleaning bias having a polarity opposite the regular charge polarity of the toner and removes the regularly charged toner from the surface of the intermediate transfer belt 901. However, a roller that applies a cleaning bias having the same polarity as the toner to the cleaning rotating member such as the cleaning brush roller 903 and removes the oppositely charged toner from the intermediate transfer belt 901 can cause the same problem.